This invention relates to a vehicle transmission control system, in particular an electronic control system for a powershift transmission having a limp-home capability.
In an electrohydraulically controlled power shift transmission (PST), proper function is dependent on several components. These components include the operator controls (e.g. shift lever and associated parts), the microcomputer-based transmission controller, the electrohydraulic valves, the wiring and the transmission elements. If any of these were to fail, the operator may lose at least part of the normal transmission function. In a worst case, the transmission would no longer carry torque and the tractor would not move. Loss of tractor motion is a major failure, so it is desirable to have a transmission system with a "limp-home" capability wherein limited tractor motion is provided in case of failures of the PST system with only the components which are normally sold as part of the tractor. In other words, the tractor must move under its own power with whatever function of the transmission is available.
A known transmission for a production agricultural tractor provides a limp-home function by disconnecting a wiring harness from the transmission controller and then connecting the two harness connectors together. It is believed that, with this known system, a clutch switch is normally closed which enables the limp-home function. Therefore, the clutch pedal need not be used when operating in the limp-home mode. However, the clutch pedal as well as the shift lever can be used to stop vehicle motion. When the clutch pedal is fully depressed, the clutch switch opens, thereby dropping power to a transmission control valve solenoid which is required for motion in both first forward gear and first reverse gear. Non-modulated engagement will occur when the shift lever is moved into gear with the clutch pedal released or when the clutch pedal is released with the shift lever in gear. Gear engagement with this system cannot be modulated by operation of the clutch, and therefore generates a jerk to the operator which is worse when engagement occurs at higher engine speeds. With this known system, only first forward and reverse gears are available in the limp-home mode. As a result, the tractor is unusable for continued work. It would be desirable to have a limp-home function which would permit modulated engagement and which would enable a gear which is in the working range.
Also with this known system, if the tractor stops as a result of some failure, and the operator leaves the engine running and the shift lever in gear, and then the operator swaps the limp-home connectors, gear engagement may result. Gear engagement could occur as soon as the limp-home connectors are mated. It would also be desirable to have a limp-home function which does not override start-in-gear protection and which permits operator modulation of transmission torque. Also with this known system, if the ignition key switch is turned on, and then a limp-home gear is engaged, and then the engine is started at the starter solenoid, the transmission will carry torque when hydraulic pressure is high enough.
Another known production powershift transmission also has a limp-home function. This is obtained by swapping certain connectors and then operating a momentary switch which allows one forward and one reverse gear. This limp-home circuit requires that the operator manually maintain the switch actuated in order to maintain motion. If the switch is released, the transmission disengages. This limp-home circuit is intended only for moving the tractor onto a vehicle or to a repair area and is not intended for continued working. It bypasses all electronic functions, including the shift quadrant and any motion preventive features, such as start-in-gear protection, as well as the transmission controller. This system would also allow gear engagement if the limp-home switch had failed or had been defeated such that it was stuck in the forward or reverse position.